The present application claims priority to Swedish Application SE 0001532-1, filed Apr. 27, 2000.
1. Technical Field
The present invention relates to a method of reducing emissions in the exhaust gases from an internal combustion engine having at least one cylinder supplied with an air/fuel mixture when a crankshaft of the internal combustion engine is rotated, at least one inlet valve, at least one inlet duct connecting to the inlet valve, at least one exhaust valve, at least one exhaust duct connecting to the exhaust valve, control members for controlling the opening and closing of the inlet and exhaust valves, and a piston reciprocating between a top dead-center position and a bottom dead-center position in the cylinder.
2. Background Information
It is desirable to reduce the undesirable emissions present in the exhaust gases of an internal combustion engine in order to reduce pollution of the surrounding environment and to satisfy legal requirements for internal combustion engines. The undesirable emissions present in the exhaust gases include, inter alia, carbon monoxide (xe2x80x9cCOxe2x80x9d), hydrocarbon compounds (xe2x80x9cHCxe2x80x9d) and nitrous oxides (xe2x80x9cNOxxe2x80x9d).
In order to reduce these emissions in the exhaust gases, the engine is provided with a catalytic converter that, by chemical reaction, burns the above mentioned emissions essentially completely. The chemical reaction in the catalytic converter occurs only when the catalytic converter has reached a predetermined working temperature. This working temperature is reached after a predetermined operating time of the engine. As such, when the engine is cold-started and prior to reaching its working temperature, there is no reduction of the above mentioned emissions in the catalytic converter.
There are known arrangements for heating a catalytic converter when the engine is cold-started in order to rapidly reach a desirable working temperature of the catalytic converter, thereby making it possible to reduce engine exhaust gas emissions at an early stage. In one such arrangement, an electric heating element is arranged in the catalytic converter. However, this arrangement makes the catalytic converter complicated and expensive to produce.
One problem with cold-starting internal combustion engines is that a comparatively great amount of fuel in relation to the air supplied, that is to say a rich air/fuel mixture, has to be supplied to the engine in order to start the engine and further so that the engine will be capable of working at an essentially constant speed during idle running. This rich air/fuel mixture is also supplied in order that the engine will be ready to provide increased torque when the accelerator is operated and in order that the engine will be less sensitive to different fuel qualities. The drivability of the engine is thus ensured before the engine has reached its operating temperature.
The absence of emission control in the catalytic converter and the rich air/fuel mixture result in the content of CO, HC and NOx emitted from the engine being high when the engine is cold-started.
Attempts have previously been made to reduce the quantity of fuel in relation to the air supplied, i.e., run the engine with a leaner air/fuel mixture when the engine is cold-started. These attempts have caused the engine to run rough when idling and negatively affects the drivability of the engine. The reason why the engine speed varies during idle running is that the torque generated by the engine is very sensitive to variations in a lambda value of the air/fuel mixture supplied to the cylinder space of the engine when the air/fuel mixture is lean. The lambda value, or excess air factor, is the actual air quantity supplied divided by the air quantity theoretically necessary for complete combustion. If the lambda value is greater than 1, the air/fuel mixture is lean, and if the lambda value is smaller than 1, the air/fuel mixture is rich.
Fuel supplied from a fuel injection valve can be controlled accurately by the fuel injection system of the engine in order to obtain a substantially constant lambda value for the air/fuel mixture supplied. However, when the engine is cold, fuel will condense on the comparatively cold walls in the inlet duct and in the cylinder. The fuel condensed on the walls will be vaporized and accompany the air/fuel mixture which is flowing in the inlet duct and being supplied to the cylinder space. If there is an uneven vaporization of the fuel condensed on the walls due to, e.g., pressure variations, temperature gradients, or the flow rate of the air/fuel mixture in the inlet duct, the lambda value of the air/fuel mixture supplied to the cylinder space will vary.
As the torque generated by the engine varies during idle running when cold-started, the speed of the engine varies. In this regard, the speed of the engine means the speed of rotation of the crankshaft of the engine. When the speed varies, the pressure in the inlet duct also varies, leading to vaporization of the condensed fuel varying, resulting in a variation of the lambda value of the air/fuel mixture supplied to the cylinder space. This intensifies the uneven speed of the engine.
When fuel supplied to the cylinder comes into contact with the cylinder walls, the fuel condenses. The fuel condensed on the cylinder walls is difficult to ignite during the expansion stroke, resulting in a great quantity of uncombusted fuel that accompanies the exhaust gases. The fuel condensed on the cylinder walls also contributes to the increased formation of HC during the combustion process in the cylinder. This negative effect increases during warming-up of the internal combustion engine, before the engine has reached its working temperature.
At the beginning of this warming-up of the engine, as mentioned above, the catalytic converter has not yet reached its working temperature. This results in the HC emitted reaching an unacceptably high level.
One object of the present invention is to reduce carbon monoxide, hydrocarbon compounds and nitrous oxides in the exhaust gases from an internal combustion engine when cold-started.
Another object of the present invention is to bring about increased after oxidation of all HC during and after the expansion stroke.
A further object of the present invention is to reach the working temperature of the internal combustion engine as rapidly as possible.
This is achieved by a method for reducing emissions in exhaust gases from an internal combustion engine wherein a lean air/fuel mixture is supplied to the cylinder, the internal combustion engine is controlled so that it works at high load, and the exhaust valve is controlled so that it opens when the piston is located in the bottom dead-center position.
By supplying a lean air/fuel mixture to the cylinder, the total amount of emissions in the exhaust gases emitted from the internal combustion engine is reduced. By controlling the engine so that it works at high load, condensed fuel on the walls of the inlet duct will have little effect on the mixing ratio between the air and the fuel, resulting in the lambda value of the air/fuel mixture supplied to the cylinder space remaining substantially constant. The crankshaft will thus rotate at a substantially constant speed while idling. By controlling the exhaust valve so that it opens when the piston is located in the bottom dead-center position, the expansion and the combustion process will go on substantially throughout the stroke volume of the cylinder. This means that fuel condensed on the cylinder walls during the induction stroke and the compression stroke is afforded the opportunity over a relatively long period of time of being burnt by the fuel flame present in the cylinder during the expansion stroke. At the same time, hydrocarbon compounds formed in the cylinder will also be oxidized during the relatively long combustion process.